We propose to create a genome-wide haplotype map in 47 frequently used inbred lab strains of mice in order to enable efficient positional cloning and genotype-phenotype correlation studies. Recent data has established that the genomes of commonly used inbred lab mice, the primary mammalian model system, are simple mosaics of long segments from a very small number of distinct Mus musculus sub-species. By providing a complete description of ancestral segments of variation in each modern strain, the map will allow the use of phenotype data from many strains and QTL mapping information from all crosses simultaneously in the fine mapping of genes responsible for medically relevant phenotypic differences. [unreadable] [unreadable]